In the Ugine-Sejournet pipe-making process, glass is used as a lubricant for hot extrusion. This Ugine-Sejournet pipe-making process is excellent in workability in pipe making, and therefore a billet can be processed at a relatively high reduction rate by this pipe-making process. Therefore, the Ugine-Sejournet pipe-making process has been used frequently in manufacturing a seamless pipe made of a hard-to-work material such as a high alloy as the source material.
A hollow billet is used as a starting material for a hot extruded pipe to be manufactured by the Ugine-Sejournet pipe-making process, and a guide hole is formed in this hollow billet in advance by machining. At the time of hot pipe making, a mandrel of an extrusion press is inserted into this guide hole.
In the case where a large-diameter mandrel is used to extrude the hollow billet for making a pipe, a pilot hole must be drilled (machined) for the billet according to the mandrel diameter. The processing of the pilot hole significantly decreases the operation efficiency, and also aggravates the yield loss. Usually, in the Ugine-Sejournet pipe-making process, the billet for pipe making to be fed to the extrusion press is subjected to hot expanding piercing as preliminary working.
FIG. 1 is an explanatory view for explaining a hot expanding piercing step performed as preliminary working of the billet for pipe making. FIG. 1A shows a state in which the hollow billet is put in place in a container. FIG. 1B shows a process in which the hollow billet is being expansion-pierced. FIG. 1C shows the hollow billet after expansion-piercing.
As shown in FIG. 1A, a hollow billet 1 with a guide hole 1a drilled therein is inserted into a container 2 in such a manner that the bottom part of billet is in contact with the inner surface of the back end of the container 2. The hollow billet 1 is heated to about 1100 to 1200° C. before being inserted into the container 2.
Successively, as shown in FIGS. 1B and 1C, the hollow billet 1 is expansion-pierced and the inside diameter thereof is expanded by using a mandrel 4 provided with a plug 3, which is used for expansion and has a predetermined diameter, at the front end thereof. The hollow billet 1 having been expansion-pierced (this hollow billet 1 is used as a billet 1 for pipe making) is pushed upward by an ejector from the bottom and is taken out of the container 2. Subsequently, the billet 1 for pipe making is reheated and fed to the extrusion press used in the Ugine-Sejournet pipe-making process.
In the hot expanding piercing step shown in FIG. 1, in the case where the hollow billet is made of a hard-to-work material, defects such as tool seizing and scratches caused by poor lubrication are very likely to occur on the outer surface and/or the inner surface of the hot pierced billet. Such a hard-to-work material may include: a high alloy of high Cr and high Ni, where containing alloying elements in large amounts; an alloy steel containing Ti; and the like.
As the prior art for preventing the defect caused by poor lubrication from occurring, the methods described below have been disclosed.
Non Patent Literature 1 describes a method in which glass is used as a lubricant in the Ugine-Sejournet pipe-making process. For example, glass powder and/or a lump-like glass powder is supplied, as a lubricant, to the beveled part on the billet upper end and the inner hole (guide hole) of the billet.
Patent Literatures 1, 2 and 3 describe a method in which a glass lubricant is coated onto the outer surface and the inner surface of the billet in the hot expanding piercing of the hollow billet used in the Ugine-Sejournet pipe-making process. As a workpiece, high-alloy Hastelloy C276 is processed in Patent Literature 1, a high-nickel alloy is processed in Patent Literature 2, and a SUS304 material is processed in Patent Literature 3.
Concerning the specific shape of glass lubricant, for example, FIG. 1B of Patent Literature 1 shows a ball-shaped glass lubricant. This ball-shaped glass lubricant is placed in the beveled part on the upper end of the hollow billet. Also, FIG. 4A of Patent Literature 1 shows a glass lubricant (formed glass) formed into a ring shape. This ring-shaped glass lubricant is placed on the upper end of the hollow billet 1. The formed glass is the one formed by solidifying glass powder with a binder such as silicate soda
In the hot piercing of the hard-to-work material, sufficient lubrication cannot be provided by merely coating glass powder or placing the lump-like glass in the beveled part on the upper end of the hollow billet. In particular, the inner surface of the hollow billet cannot be sufficiently lubricated. In some cases, therefore, there is used a method in which the formed glass of a ring shape is placed on the upper end of the hollow billet (example: the formed glass shown in FIG. 4A of Patent Literature 1).
However, in the case where the ring-shaped formed glass is merely placed on the billet, the glass is dislocated or scattered by vibrations etc during expansion-piercing, and/or by the change of position of the plug. Therefore, the lubrication may become insufficient, and eccentric wall happens to occur in the billet after expansion-piercing.
Conventionally, at the time of hot expanding piercing, a metallic ring has been mounted around the ring-shaped formed glass on the billet upper end (refer to FIG. 2B described later). By the mounting of this metallic ring, the glass can be prevented from dislocation (misfit) or scattering.
However, the method in which the metallic ring is mounted around the ring-shaped formed glass has the following problems:
(1) Mounting the metallic ring before hot expanding piercing and demounting it after the completion of expansion-piercing are performed in a high-temperature environment. Therefore, it is difficult to ensure the worker's safety.
(2) The inner surface of the hollow billet cannot be sufficiently lubricated by merely placing the glass lubricant on the upper end of the hollow billet. Therefore, defects may occur on the inner surface of the billet after hot expanding piercing.